I’m not sure how or why grape became the default. For the longest time, I thought I didn’t like PB&J. But ever since trying it with strawberry instead, I understood the appeal. It’s so much better, and now I find it weird that the combination isn’t more common.
There really is no wrong way. Peanut butter, or something like it, whatever you’re into, with some pulverized fruit jelly stuff, on some sort of bread or breadlike surface, open faced or sandwich style. All good.